1. Field of the Invention
The present invention generally relates to lasers, and particularly to lasers in which a fundamental laser emission at a first (fundamental) frequency is converted to a second (converted) frequency.
2. Description of Related Art
A typical laser comprises an optical cavity, a gain medium situated within the cavity, and a pump source that pumps the gain medium to a level at least sufficient to support lasing operation and generate a laser emission at a fundamental wavelength. In a frequency-converted laser, the fundamental laser emission propagates through a nonlinear material arranged to frequency-convert the fundamental wavelength to a second wavelength. Often, the frequency-conversion is a doubling process in which the frequency is doubled (i.e. the wavelength is halved); however other processes such as frequency-tripling or sum-frequency processes are used in some lasers.
Frequency conversion in lasers is an important process because it allows access to laser wavelengths that would otherwise be inaccessible or difficult to generate. For example, many commercial laser products frequency-double the infrared 1064 nm line in a Nd-doped solid state material such as Nd:YAG or Nd:YVO4 thereby generating a green beam at 532 nm. However, it can be difficult to efficiently convert the frequency and/or obtain high power in a frequency-converted laser.
Desirable objectives of a commercial laser product are high efficiency, stable output, high beam quality, long operating lifetime, and reasonable cost. Unfortunately, many tradeoffs are made to obtain stable operation at high power and at a reasonable cost. For example, high conversion efficiency is obtained when the nonlinear material is situated within the optical cavity where the fundamental emission has a higher intensity than outside the optical cavity. However, this intra-cavity arrangement creates a host of other problems such as instability caused by the frequency-converted radiation disrupting the laser emission process within the optical cavity.
In order to make a frequency-converted laser more stable and prevent propagation of the frequency-converted beam through the gain medium, it has been suggested to utilize a folded-cavity configuration in which a folding mirror, which is highly reflective of the fundamental laser emission, is situated between the gain medium and the nonlinear material. The folding mirror is highly transmissive of the second wavelength, while the end mirror is highly reflective of the second wavelength, and therefore the frequency-converted beam exits through the folding mirror before propagating through the gain medium. However, the folded cavity configuration is difficult to align, it has proven difficult and costly to make in commercial quantities.
Japanese Patent Application Disclosure No. Hei 1-239892, by Gotoh, entitled xe2x80x9cSolid State Laser Devicexe2x80x9d discloses a linear cavity that includes a mirror mounted to form Brewster""s angle to the axis of resonance, enabling the fundamental (basic) emission at a specific linear polarization to pass through the Brewster""s angle mirror, while providing a high reflection at Brewster""s angle for orthogonally-polarized light at the second harmonic wavelength. The Brewster""s angle mirror is mounted between a nonlinear crystal and a laser medium, and includes coatings that enhance transmission of the polarized fundamental emission at Brewster""s angle. Additionally, the coating must increase the reflection of orthogonally-polarized second harmonic wavelength at Brewster""s angle. Unfortunately, the Brewster""s angle mirror requires linear, orthogonal polarizations of both the fundamental emission and the second harmonic beam, which can lead to reduced efficiency. Furthermore, this polarization requirement dictates that the laser must be precisely aligned for both the fundamental emission and the second harmonic beam, which is difficult and costly, because even a minor polarization misalignment can significantly reduce power and efficiency. Misalignment can occur for a number of reasons, such as stress- or temperature-induced birefringence in the nonlinear crystal or gain medium. If the main polarization of the fundamental emission is misaligned, then the angled mirror will reflect some of the fundamental emission out of the cavity, preventing laser operation. If the main polarization of the second harmonic beam is misaligned, then some of the second harmonic beam is not reflected out from the cavity. In either instance the frequency-converted laser suffers reduced efficiency, and thus the laser configuration with the Brewster""s angle mirror is not desirable, especially given the expected lifetime of a commercial product.
Marshall, in U.S. Pat. No. 5,511,085, entitled xe2x80x9cPassively Stabilized Intracavity Doubling Laserxe2x80x9d discloses several embodiments of an intracavity-doubled laser, including an intracavity frequency-doubled laser in a linear configuration. In some embodiments described by Marshall, a linear cavity laser includes an intracavity dichroic mirror placed between the laser crystal and the nonlinear frequency-doubling crystal, the intracavity mirror being highly reflective at the doubled frequency but transmissive at the fundamental. The intracavity mirror reflects the backward-propagating frequency-doubled light directly back through the nonlinear crystal at 180xc2x0 so that both the forward frequency-doubled beam and the backward frequency-doubled beam exit the cavity through the end mirror, overlapped as a single beam propagating in the same direction. Unfortunately, reflecting the converted beam directly back along the beam path, collinear with the forward-propagating converted beam, has been found to adversely affect the efficiency and stability of the frequency conversion process, creating undesirable output power fluctuations and low average power.
In order to overcome the limitations of prior art solid state lasers, the present invention provides an efficient frequency-converted solid state laser that outputs a single beam utilizing an angled reflector situated within the laser cavity.
The frequency-converted laser comprises an optical cavity including a first reflector and a second reflector that define an optical axis. The first and second reflectors are reflective at the first wavelength, the second reflector also being reflective at the second wavelength. A gain medium is situated within the optical cavity and a pump source is arranged to pump the gain medium to excite the laser emission within the optical cavity. A nonlinear material is situated between the gain medium and the second reflector, the nonlinear material being arranged within the optical cavity to convert fundamental wavelength of the laser emission to a second wavelength. An angled reflector that is reflective of the second wavelength and transmissive of the first wavelength is situated within the optical cavity between the first reflector and the nonlinear material, arranged to reflect optical radiation at the converted wavelength at a nonzero exit angle with respect to the optical axis.
In operation, the fundamental laser emission generated in the gain medium at a first wavelength is frequency-converted to second wavelength in the nonlinear crystal. Particularly, the fundamental laser emission includes a forward-propagating emission propagating in a first direction and a reverse-propagating emission propagating in a second, opposite direction. The forward-propagating emission is frequency-converted to generate a first converted beam propagating in the first direction, which is then reflected from the second reflector. The reverse-propagating emission is frequency-converted in the nonlinear material to generate a second converted beam that co-propagates in the second direction together with said reflected first converted beam to provide a combined frequency-converted beam. The combined beam is reflected at a predefined nonzero angle with respect to the optical axis to provide a reflected converted beam. Typically, the angled reflector and the exit angle are selected so that the angled reflector reflects substantially all polarizations of the combined beam, and the exit angle is a non-Brewster""s angle.
In some embodiments, the nonlinear material comprises a first end proximate to the gain medium and a second end proximate to the second reflector, and the angled reflector is formed on the first end. In some such embodiments, the second end comprises a first section that intersects the laser axis, the first section being substantially reflective at the second wavelength and a second section arranged with respect to the angled reflector so that the reflected combined converted beam is output therethrough, the second section being substantially transmissive at the second wavelength.
In other embodiments, the gain medium is coupled to a surface that comprises the angled reflector. In some such embodiments, the laser further comprises an optically transparent heatsink coupled to the gain medium, the heatsink including the surface that comprises the angled reflector. Some embodiments include means for reflecting the first converted beam from the second reflector so that the reflected first converted beam is approximately in phase with the second converted beam within the nonlinear material.